This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A116015 #5 Oct 15 2013 10:31:53 %S A116015 1,9,43,109,119,161,175,192,232,490,843,1050,1162,1242,1635,1959,2113, %T A116015 2859,2935,3008,3174,3469,3717,4165,4173,4761,4839,4876,5200,6111, %U A116015 6423,6468,8008,8480,9064,9523,9603,11143,11197,11353,11557,11773 %N A116015 n+sigma(n)+sigma(sigma(n)) is a triangular number. %e A116015 6111+sigma(6111)+sigma(sigma(6111))=T(286). %t A116015 triangularQ[n_] := IntegerQ[Sqrt[8n + 1]]; Reap[For[n = 1, n < 12000, n++, s = DivisorSigma[1, n]; If[triangularQ[n + s + DivisorSigma[1, s]], Sow[n]]]][[2, 1]] (* _Jean-François Alcover_, Oct 15 2013 *) %Y A116015 Cf. A116012, A116013, A116014. %K A116015 nonn %O A116015 1,2 %A A116015 _Giovanni Resta_, Feb 13 2006